Building a planet-sized telescope
This article first appeared in the 2019 Physics World Focus on Instruments & Vacuum
At times, Gopal Narayanan admits, putting together the telescope that captured the first image of a black hole was an “organizational–logistical nightmare”. Narayanan is an astronomer working at Mexico’s Large Millimeter Telescope (LMT) Alfonso Serrano, which is one of eight ground-based radio observatories that make up the Event Horizon Telescope (EHT). Narayanan is therefore well-versed in the project’s complexities, from high-level discussions between collaborators down to the many layers of equipment that made the image possible. The fact that this group of 250 or so scientists and engineers pulled off such a tricky observation is, he says, “a testament to human ingenuity”.
That ingenuity comes into sharper focus when you consider how many different technologies had to work together before the black hole could come out of its shadow. The idea of bringing together existing radio telescopes operating at millimetre wavelengths originated in the mid 2000s, and rests on a concept known as Very Long Baseline Interferometry. The “very long baseline” part refers to the large distances separating the observatories: the EHT’s other telescopes are located in Hawaii, Arizona, Chile, Antarctica and Spain. The “interferometry” part is what enables the collaboration to decode detailed images from the radio signals that each telescope receives. The astronomers combine signals from the different telescopes, creating constructive interference from which meaningful signals emerge. In doing so, they create what Narayanan calls “the next best thing to an Earth-sized telescope”.
But it’s a strange kind of telescope. Each observatory is unique, and therefore faces highly individual challenges, like overcoming local conditions to point to the right place in the sky. Yet to become a successful interferometer, they also must strive to record data in harmony, meaning all must use similar maser clocks and digital electronics.
Interferometers seek to add together amplitudes of desired signals, explains Miguel Sánchez Portal, station manager of the EHT’s Spanish member, the Instituto de Radioastronomía Milimétrica (IRAM) in Granada. Within an interferometry network, each telescope’s receiver collects signals from radio waves, for example as voltages from the sensors in its receivers. Later, the continuous recorded signals go into a system that correlates the data from different observatories. EHT has two correlator centres, one at the Max Planck Institute for Radio Astronomy in Bonn, Germany, and one at the Massachusetts Institute of Technology’s Haystack Observatory near Boston, US. These correlators shift the observatories’ data sets until they add coherently, increasing the wave’s amplitude, Sánchez Portal explains.
Before the signals can be added together with any confidence, each telescope needs to be able to point very precisely at whatever celestial target the collaboration wishes to observe. For the EHT’s largest telescopes, this is literally a tall order. When the LMT helped record the iconic image, its primary reflector surface was 32 m in diameter – half the height of the famous pyramids at Teotihuacán, a few hours’ drive away. The LMT also sits 4600 m above sea level, on a mountain, Sierra Negra, that is frequently exposed to buffeting winds. To make matters worse, changes in the amount of solar radiation hitting the LMT over the course of a normal day and night makes its supporting structure curl “almost like a potato chip”, Narayanan says.
To make the telescope both stiff and routinely able to return to the same position, LMT’s scientists and engineers must fight the elements. The telescope’s primary reflector is currently composed of 180 plates, all of which require micrometre-range adjustments. To accomplish this, each plate ...
More on: physicsworld.com